Method and apparatus for testing pulsatile endurance of a vascular implant

ABSTRACT

A method for testing the pulsatile endurance of a vascular implant  3  comprises placing a resilient insert  4  into the implant and repeatedly expanding and contracting the insert, thereby expanding and contracting the implant. The insert preferably has a cavity therein and is repeatedly expanded and contracted by repeatedly increasing and decreasing the pressure in the cavity.

The present invention relates to a method and apparatus for testingpulsatile endurance of a vascular implant.

Prosthetic vascular implants, such as heart-valves, stents, grafts andstent-grafts used for human implantation are subjected to the continuousfluctuating stress of blood pressure. It is therefore necessary to testsuch implants to prove their durability over a lifetime of exposure topulsatile blood pressure.

A number of prior art documents disclose destructive methods of testingnon-resilient vessels (such as glass bottles) by inserting a resilientinsert such as a bladder into the vessel and subjecting the bladder toextremely high expansive pressure to see if the vessel breaks (see forexample U.S. Pat. No. 3,895,514; GB 2,177,220; GB 1,531,557; and GB2,149,126). None of these methods would be suitable for the pulsatiletesting of a vascular implant.

Commercial machines for pulsatile testing of vascular implants areavailable from suppliers such as Enduratec Inc and Dynatek Dalta. Theseprovide one or more resilient tubes into which the implant is placed.The tubes are filled with liquid, typically isotonic saline, and thepressure within the tube is varied by means of a pump. Different typesof pump are used, some workers employ positive displacement mechanicalpumps while others prefer electrically driven linear motors which drivepistons directly.

The fatigue process relies upon a first raised liquid pressure insidethe tube expanding the tube and a second, lowered, liquid pressureallowing the tube to contract. As the tube expands, the radialresilience of the implant causes it to expand with the tube. As the tubecontracts, it squeezes the implant back to its original size.

A similar method is disclosed in DE 199 03 476 (Inst.Implantatechnologie) which relates to a method of testing blood vesselimplants by placing them inside an elastic sheath and subjecting them toexternal pressure.

There are a number of common failures or difficulties associated withlocating the implant within a tube.

The most serious commercially is the consequence of a tube rupturingduring the test. In most circumstances, a catheter or similar tube isemployed to insert the implant into the tube. The implant is firstcrushed before passing through the catheter and this crushing processcan severely affect the life expectancy of the implant. Thus, if thetube fails during a test, the tube itself can be replaced but it is notfeasible to re-deliver the implant through a catheter. This is becauseit would involve crushing the implant into a catheter a second time andits life expectancy will consequently be reduced. The cost of replacingthe implant is rarely important. However, endurance tests typically lastbetween 3 and 6 months and such a failure can easily delay testing, andtherefore the time to launch a product, by several months.

As a consequence of the above failure mode, designers of test machineswill usually employ a particularly tough tube with thick walls. Thecompliance of such a tube (i.e. the percentage increase in diameter perunit pressure) is relatively low, and in order to achieve changes indiameter which are physiologically representative, the pulsatilepressures used to inflate the tubes are usually significantly higherthan physiological pressures. For instance, in the abdominal aorta,blood pressure in the average healthy subject is 120 mm Hg/80 mm Hg,i.e. the blood pressure varies by 40 mm Hg for every pulse. Complianceof a healthy aorta can be of the order of 5% per 100 mm Hg so that achange in diameter of 2% can be expected at every heart beat. In orderto simulate such a change in diameter, some workers employ a pulsepressure between 80 mm Hg and 100 mm Hg.

If the implant presents a significant surface area across the lumen ofthe vessel, such as a tapered stent or stent graft, then the force perunit area along the axis of the implant is increased in proportion withthe inflation pressure of the tube. This elevated pressure inducesfailure modes such as limb separation or migration which would not occurat physiological pressures.

A shortcoming of existing designs unrelated to the failure describedabove lies in the limitation of the form of the tube. Stent graftsfrequently are designed for use in bifurcated vessels and requirebifurcated test tubes for their testing. Stent-grafts are also intendedfor use in aneurysms. Accordingly, where the vessel is normal, parts ofthe implant will be in contact with the wall of the vessel, whereaswhere the vessel is aneurysmal, the implant will be passing through avoid. Moreover, diseased vessels are frequently highly tortuous. As aresult of these factors, tubes must be available that bifurcate, thathave different compliance in different places, that can be aneurysmaland which are highly tortuous. The production of such complex tubes is,even where possible, difficult, expensive and time consuming.

A further issue in endurance testing arises from the need to completelife-time tests in a commercially appropriate period of time. Typically,vascular implants are tested for 400,000,000 cycles which representapproximately 10 years of implantation life at a heart rate of 80 beatsper minute. Many companies test large implants at approximately 35 Hz,allowing testing to be completed in approximately 19 weeks.

It is desirable to increase the speed of testing by as much as possiblein order to accelerate the time taken to bring a new product to market.However, the testing method described above has a frequency limit whicharises from the radial resilience and the surface area of the implant.This arises from the following mechanism:

When the pressure in the tube is increased, it moves away from the wallsof the implant. The radial resilience of the implant causes the wall ofthe implant to follow the wall of the tube. However, this resilience maynot be sufficient to overcome the frictional drag of the fluid throughwhich the implant wall must move and the implant wall is likelytherefore to move more slowly than the wall of the tube. Thus, where thedrag is high, the radial resilience low and the testing speed is alsohigh, the vascular implant can lag behind the movement of the wall ofthe tube. In these circumstances, the strain induced in the implantreduces as the frequency increases and the change in diameter of theimplant no longer matches the change in diameter of the tube.

The present invention relates to an improved arrangement for testingvascular implants which overcomes or minimises all of the limitationsdescribed above.

In a first aspect of the present invention, there is provided a methodfor testing pulsatile endurance of a vascular implant, comprisingproviding a resilient insert, inserting the insert into the vascularimplant, and repeatedly expanding and contracting the insert, therebyexpanding and contracting the implant.

Although the insert may be such as to be expanded and contractedmechanically (for example it may comprise an expandable stent), itpreferably has a cavity therein and is repeatedly expanded andcontracted by repeatedly increasing and decreasing the pressure in thecavity. The preferred frequency of expansion/contraction is at least 25Hz, more preferably from 25 to 100 Hz, most preferably from 50 to 100Hz.

The improved technique preferably employs a tube which is deployedinside the vascular implant, the tube being made of a resilient materialsuch as latex rubber, silicone rubber, poly-urethane or similar.Preferably, the tube is made with very thin walls so that inflationpressures within the tube are transferred directly to the inner surfaceof the implant under test. In practice, contraceptive condoms provide anideal tube for testing larger implants.

Such an arrangement has the advantage that should the tube break duringthe test, a replacement tube can be threaded into the implant withoutrisk of damage to the implant. In this way, failure of a tube will neverautomatically require the test implant to be rejected nor lose thetesting time up to the moment of failure.

A second benefit of such an arrangement is that physiological pressurescan be used within the tube because there is very little attenuation ofthe pressure by the very thin walls.

A third benefit of such a system is that the mechanical properties ofthe vessel surrounding the implant under test can be varied at differentpoints and the vessel can even be made of separate components becausethere is no longer a requirement that the outer tube be fluid-tight.This allows the compliance of different regions to be optimised withoutthe requirement that the entire ‘vessel’ is made from the same material.

A fourth benefit of the such a system is that the internal tube is verysoft and this permits the test implant to be bent or angled severely,purely by means of restraints, rather than requiring a custom made,angled tube.

A fifth benefit of the system allows the test frequency to be increasedbecause the implant is driven internally to expand rather than relyingupon its radial resilience. When used in combination with an outer tube,the above described system provides a positively driven method ofexpanding an implant and additional resilience from the outer tube tocompress an implant. The movement of the wall of the implant is thenmuch less dependent upon the characteristics of the implant alone andtesting can be carried out at frequencies of 50 Hz to 100 Hz. At thisspeed, testing to 400 million cycles can be completed in 7 weeks.

The diameters of implant that can be accommodated by such a machine liein the range 2 mm to 50 mm, although if having sufficiently thin walls,the inner test tube can be significantly under- or over-sized.

The wall thickness of the inner tube preferably lies in the range from0.03 mm to 0.2 mm, although with loss in performance, some benefits ofthe inner tube can still be gained if the wall thickness is severalmillimetres.

A preferred method of obtaining high frequency expansion and contractionof the insert is to employ a modulator such as a rotating valve oroscillating piston to modulate a continuous supply of air into a seriesof pulses of the required frequency.

A preferred embodiment of the invention will now be described withreference to the drawing, in which:

FIG. 1 illustrates an arrangement of apparatus set up to test bifurcatedimplants in accordance with the invention.

The apparatus of FIG. 1 comprises:

A supporting gantry (1).

Inlet tubes (2).

Vascular implant (3).

Inner tubes (4).

Short outer tube to reduce compliance at the neck of the implant (5).

Bungs (6) and (7).

This arrangement employs ultra-thin walled condoms as inner tubes (4)used as a pair to fill the single main body of the implant (3) and itstwin legs. In order to allow higher pressures to be used within thetubes (4), bungs (6) and (7) are used to limit the extent to which eachtube can expand length-wise. At each exit to the vascular implant (3),this limit is arranged to lie within a portion of outer tube which runscontinuously to the vascular implant. In this way, there is no path forthe inner tube to expand or herniate beyond the vascular sample oroutside the outer tube. This limits the ultimate strain put on the innertube and prevents it from bursting unless very high pressures areemployed.

A further improvement employed in this arrangement is the use ofcompressed air as the pressurising medium for the inner tubes. In orderto make the air pressure pulsatile, a rotating valve or oscillatingpiston can be used and the design of such a valve or piston is greatlysimplified by only being required to modulate the pressure of air. Otherworkers using saline filled systems generally require the pressuremodulator to operate directly on salt water which involves the problemsof corrosion and leakage.

The rotating valve (known as a “pulser”) consists of a cylindricalhousing into which is fitted a rotating cylinder. The cylinder has twoholes through it, perpendicular to its main axis of rotation. These areperpendicular and axially displaced with respect to each other along theaxis of rotation. The housing has two parallel holes through it, whichare axially aligned with the holes in the cylinder.

As the cylinder rotates, the transverse holes become alternately alignedand misaligned with those in housing. When aligned with the inlet tube,pressure is transmitted to the condom. When aligned on the exhaustconnection, pressure is released. Thus, as the cylinder rotates, thecondom is repeatedly pressurised and depressurised. The pressure pulsemay be adjusted by changing the initial air pressure, the size of theexhaust port and the speed of rotation of the cylinder.

A further benefit of employing air to pressurise the system is that themass of oscillating fluid is significantly reduced compared to usingsaline solution. This in turn reduces the power required of themodulating system.

In order to employ an air pressurised system, it is still preferred thatthe vascular implant is maintained at physiological temperatures and insaline. Where the outer tube is discontinuous, the implant can be keptin saline by placing the entire system in a bath of salt water at anappropriate temperature.

In large implants, the change in volume per pressure pulse can be largeand this places significant demands on the modulator. Compressed airsystems are more demanding than liquid-filled systems in this respectbecause of the compressibility of the gas. In order to reduce the volumeof gas in such a system, the inner tubes can be part-filled with waterand small bore tubes can be used.

1-21. (canceled)
 22. A method for testing pulsatile endurance of avascular implant comprising: a. providing a resilient insert, b.inserting the insert into the vascular implant, and c. repeatedlyexpanding and contracting the insert, thereby expanding and contractingthe implant.
 23. The method of claim 22 wherein: a. the insert has acavity therein, and b. the insert is repeatedly expanded and contractedby repeatedly increasing and decreasing the pressure in the cavity. 24.The method of claim 23 wherein the walls of the insert surrounding thecavity are from 0.03 to 0.2 mm thick.
 25. The method of claim 23 whereinthe pressure in the cavity is increased by supplying the cavity with afluid under pressure.
 26. The method of claim 25 wherein the fluid isair or saline solution.
 27. The method of claim 22 wherein the insert isa flexible tube which is closed at one end.
 28. The method of claim 22wherein the insert is formed from one of the following materials: a.latex rubber, b. silicone rubber, or c. polyurethane.
 29. The method ofclaim 22 wherein the insert comprises a contraceptive condom.
 30. Themethod of claim 22 wherein the frequency of expansion and contraction ofthe insert is from 50 to 100 Hz.
 31. The method of claim 22 wherein theimplant is at least partially immersed in saline solution duringexpansion and contraction.
 32. The method of claim 22 wherein: a. theimplant is a furcated graft having two or more branches extending from ajuncture, and b. two or more of the inserts are employed, at least oneinsert being situated in each branch of the bifurcation.
 33. The methodof claim 22 wherein the implant is a vascular graft with an internaldiameter from 2 to 50 mm.
 34. The method of claim 22 wherein the stepsof claim 22 are carried out continuously over a period of about 7 weeks.35. The method of claim 22 wherein the contraction of the implant is dueonly to its inherent resilience.
 36. The method of claim 22 wherein: a.a resilient outer sheath is provided, the implant being at leastpartially located in the sheath so that the implant presses against thesheath during the implant's expansion, and b. the resilience of thesheath provides a compressive force to the implant.
 37. The method ofclaim 36 wherein the sheath is formed of the same material as theinsert.
 38. A device for testing pulsatile endurance of a vascularimplant comprising: a. a resilient insert having a cavity therein, b.means for repeatedly increasing and decreasing the pressure in thecavity in order repeatedly to expand and contract the insert, therebyrepeatedly expanding and contracting the implant into which, in use, theinsert is inserted.
 39. The device of claim 38 wherein the insert isflexible tube which is closed at one end.
 40. The device of claim 38wherein the means for repeatedly increasing and decreasing the pressurein the cavity can provide a frequency of expansion and contraction ofthe insert of from 50 to 100 Hz.
 41. The device of claim 38 wherein themeans for repeatedly increasing and decreasing the pressure in thecavity is a source of compressed air.
 42. The device of claim 38additionally comprising a resilient outer sheath in which the implant isat least partially located, the sheath providing a compressive forcewhen the implant expands against the sheath.
 43. A device for testingpulsatile endurance of a vascular implant comprising a resilient inserthaving a cavity therein, wherein: a. the vascular implant is fit aboutthe resilient insert, and b. the insert is repeatedly flexed by pressurevariations in the cavity, wherein the insert bears against the interiorof the vascular implant during flexure.